Instruments for detecting gases of the above-mentioned type operate on the principal of drawing a sample stream to be analyzed through a heated bed of mercuric oxide where certain reducing gases in the stream react with the bed to produce free mercury vapor. This free mercury vapor then is analyzed in a sample cell located downstream of the bed by means of an ultraviolet photometer centered at a wavelength of about 254 nm. Sample reactions for carbon monoxide and hydrogen are as follows: EQU CO+HgO.fwdarw.CO.sub.2 +Hg (vapor) EQU H.sub.2 +HgO.fwdarw.H.sub.2 O+Hg (vapor)
Additional reducing gases include lower aldehydes, acetylene, ethylene, propylene, formaldehyde, acetone and others. Because reactions with these gases occur very slowly at room temperature, devices of this type are generally operated at 200.degree.-260.degree. C. to enhance the degree of reactivity and to increase the sensitivity of the instruments to such gases. Unfortunately, operating at these temperatures also creates a "background" concentration of mercury vapor leaving the HgO bed due to the thermal dissociation of mercuric oxide at elevated temperatures. Certain designs of gas detection instruments aim to minimize this "background" concentration of mercury vapor since this is the basic cause of instrument output drift and noise and, as such, is the primary restriction on ultimate sensitivity.
Conventional gas detection instruments also involve a number of filtering stages upstream of the HgO bed. Since any number of suitable gases in the sample stream may react to produce mercury vapor, those not of interest must be selectively filtered out, either by trapping (molecular sieves) or by catalytic combustion (CuO, MnO and other oxides of noble metals). Additionally, a separate means must be supplied for removing the sample of interest as well, since this is necessary for establishing the "zero" reading of the instrument in the presence of the before-mentioned "background" mercury vapor concentration.
To this end, a selective catalyst is located upstream of the mercuric oxide bed, the sample stream being first circulated through the pre-filtering apparatus and then through the selective catalyst (silver oxide in the case of CO analysis) which oxidizes the sample of interest. The instrument output associated with the situation is chosen as "zero". The sample stream is then diverted past or around this catalyst, reacting directly with the mercuric oxide bed and causing an increased concentration of mercury vapor in the bed effluent. The resulting change in instrument output is then proportional to the concentration of the component of interest in the sample stream.
Problems arising from the aforesaid instrument designs involve the dependence on a number of filtering devices which must perform properly and the need for a constant sample flow rate, as variations in flow rate cause relatively large changes in background mercury vapor concentration resulting in output noise and drift. Additionally, such designs are adapted to measure only one particular gas concentration at a time, with substantial modifications being required for measurement of other gases. An example schematic of such a device is pesented in FIG. 6 herein.
An alternative technique to the above continuous sampling or "straight-through" method involves the injection of a discreet gas sample to be analyzed directly into a "carrier gas" stream which flows continuously through the detector. Such techniques are generally referred to as gas chromatography and rely on some means for separating or partitioning the various sub-components of interest in the sample in such a manner that the carrier gas stream conveys each sub-component to the detector separately and sequentially. Devices for accomplishing the above are generally long tubes or columns packed with substances which separate gases on a basis of molecular size or other chemical properties, such as solubility and polarity. Variations in gas properties cause differences in diffusion rates through such columns, which results in each gas in the sample having a characteristic retention time on the column. In this manner, each sample constituent is swept through the column and into the detector in an individual and separate manner, characteristically referred to as a "peak". The term peak is derived from the characteristic detector output resulting from the passage of such column effluents which is in the form of a sharp spike, and the height and area of which is proportional to gas concentration in the sample. A representative diagram of such an apparatus is shown in FIG. 4 herein.
There are significant differences in design criteria for detectors operating on the above-two principals. While continuously-sampling detectors have an unlimited supply of sample gas to draw from, chromatography detectors have a limited volume of sample available for analysis (standard sample volumes are generally less than 5 cc and are limited by column technology and standard practice). Consequently, such detectors should have minimum sample cell volume and minimum system volume or "dead space" as well to insure that no mixing of gases takes place downstream of the column. Additionally, chromatography detectors must have fast response times to follow changing gas concentrations which emerge from the column in rapid succession (a single peak may pass through the detector within a few seconds). Finally, chromatography detectors must be designed such that they function properly in the carrier gas flow rate range normally associated with gas chromatography (dictated by column parameters and standard operating practice). These flow rates are generally 20-60 cc/minute rather than the 500-2000 cc/minute flow rates associated with previous designs.
One problem associated with instruments of the type under consideration is that mercury vapor has a tendency to condense on surfaces downstream of the HgO bed. Such condensation not only severely retards the response time of the detector but also reduces the maximum output in peak height of such a detector operating in the gas concentration mode, as response to each peak would be spread out over a longer time interval with an accompanying decrease in peak mercury vapor concentration. This also causes a non-linear relationship between peak height and gas concentration which makes interpretation of results difficult.
Other instrument designs have attempted to solve the problem by heating the sample cell downstream of the bed to a temperature higher than that of the reaction bed itself, and to construct all portions of the apparatus which come into contact with mercury vapor from fused silica which resists surface "wetting" by mercury. While such methods may enhance response characteristics to some extent, response is still inadequate for gas chromatography purposes. Additionally, such designs are intrinsically fragile and subject to breakage. A simpler technique is the reduction in surface area of all available condensing surfaces by way of reducing the diameter of the sample cell and its associated inlet and outlet ports. The increased flow velocity obtained in such a design will also reduce the length of time that an individual mercury vapor molecule spends within the detector, minimizing the probability of a surface/molecule "interaction". To this end, unswept volumes or `dead spaces` within the detector should also be avoided as they create areas of low flow velocity. With conventional gas chromatography flow rates, significant decreases in response time have been obtained with sample cell diameters of 0.16 cm and volumes of 0.20 cc versus previous designs having diameters of 3.0 cm. and volumes of 200 cc. Such small volumes are also good practice in terms of the before-mentioned small sample sizes involved in gas chromatography. It should also be noted that such means will not adversely affect the sensitivity of the photometric mercury vapor determinations, as this is dependent on the length of the sample cell light path rather than the sample cell cross-section area. This is also good design practice in terms of the before-mentioned small sample sizes involved in gas chromatography.
Other available condensation surfaces which have been overlooked by previous instrument designers include the HgO bed containment surfaces themselves. The glass wool and porous plugs utilized in other designs have large surface area-to-volume ratios and, while they are suitable for continuously-sampling detectors, they severly limit the response of a gas-chromatography detector.
The large reactive beds of earlier designs, with their correspondingly large surface areas relative to flow velocity through the bed also retard passage of mercury vapor through the bed in a similar manner. Another shortcoming of previous designs is the failure to insure the uniform distribution of flow across the bed cross-sectional area. Designs incorporating bed containment means comprised of glass wool plugs or other means having irregular rather than sharply-defined flat surfaces which result in variations in bed depth along the flow path. These, in turn, create irregularities in flow velocity through the bed and subsequent zones of low or zero flow which equilibrate slowly with changes in concentration.
Non-uniform bed flow is also a result of variations in both the size and shape of the HgO particles comprising the bed. The varying packing density obtained from such partial size distributions leads to porosity differentials across the bed cross-section. Regions of smaller-sized particles are less porous and, therefore, serve to restrict the passage of bed flow to a greater degree than areas containing larger-sized particles. It should be noted that relatively large HgO particles are both mechanically and thermally fragile due to their crystalline structure and amorphous shapes. Reactive beds which are initially uniformly-packed with such particles may, therefore, exhibit significant increases in time response and other negative operational characteristics when subjected to pressure, mechanical vibration, or thermal cycling which cause fragmenting or "powdering" of the bed. Comparative outputs for conventional detectors are shown in FIGS. 7 and 8. Results achievable with the present invention incorporating improvements in the negative design aspects noted previously and present in earlier designs are shown in dashed lines vs. the response of earlier designs shown in full lines.
Publications relating to gas detection instruments of the type described include the following journal articles: "A Sensitive Gas Detector Permits Accurate Detection of Toxic or Combustible Gases in Extremely Low Concentrations", Analytical Chemistry, Vol. 26, No. 9, September 1954, and "Carbon Monoxide in the Atmosphere", Journal of "The Air Pollution Control Association", 1968, Vol. 18, pages 106-110. A gas detection apparatus is made and sold by Bacharach Instrument Company, 625 Alpha Dr., Pittsburgh, Pa. 15238, the apparatus being identified as Model U.S. 400L, Ambient Carbon Monoxide Analyzer.